The patterning and morphology of a dendritic field determines the number and type of inputs that a neuron is able to receive. Consequently, dendrite defects are the strongest anatomical correlates of the intellectual disabilities manifeste in genetic disorders such as Down Syndrome (DS). In a recent screen for dendrite abnormalities in Drosophila dendritic arborization neurons, I identified minibrain (mnb) kinase as an important regulator of the cytoskeleton during arborization. Mnb is a DS critical gene that has been implicated in neuronal development, brain function and DS pathology. However, it is currently unclear what the physiological substrates of mnb are in neurons, what the molecular mechanism of mnb is during dendrite development and maintenance, and how the imbalance in mnb gene dosage might cause the neuronal and behavioral alterations associated with DS. The goal of this proposal is to answer these questions and define the mechanistic role of mnb kinase during dendrite formation, in order to understand how this pathway contributes to neuronal development and function. During my graduate training with Dr. Richard Vallee at Columbia University, I combined in vitro biochemical and biophysical assays with in vivo neuronal techniques in order to decipher how mutations in the molecular motor, cytoplasmic dynein, cause neurodegenerative and neurodevelopmental diseases in mice and humans. During my postdoctoral career in Dr. Yuh-Nung Jan's lab at the University of California, San Francisco, I have developed additional skills in Drosophila genetics, molecular biology, and live imaging in intact larvae to study the cytoskeletal organization required to create specific dendritic arborization patterns in neurons. For the experiments proposed in this application, I will continue to employ these skills, and I will acquire additional training in behavioral analysis, mass spectrometry, and in vivo super resolution microscopy in collaboration with experts from each field. This training will allow me to achieve the following aims and launch an independent research laboratory within two years: 1) Identification of direct novel targets of mnb kinase using a chemical genetic approach, 2) Functional analysis of mnb and its substrates during dendrite development and maintenance under control and DS-like conditions using STORM, and 3) Characterization of the mnb pathway in Drosophila learning and memory. With the new training and information gained in the K99 phase, I will extend the scope of my research in the R00 phase. I plan to analyze the cytoskeletal interactions and the mnb phosphorylation targets during different stages of dendrite development. In addition, I plan to investigate the roles of th minibrain phosphorylation targets in Drosophila learning and memory. Finally, I will perform an enhancer/suppression screen to identify the mechanisms regulating mnb expression and activity during neuronal development. Overall, the implementation of this proposal will elucidate the pathways involved in dendrite formation during neuronal function and dysfunction and will contribute to our understanding of the mechanism of mnb in the progression of DS.